Aedes sollicitans mosquitoes became infected with Eastern equine encephalitis (EEE) from feeding upon an inoculated horse. These mosquitoes, after two weeks' incubation, transmitted infection by bite to a normal horse. Attempts to infect more mosquitoes by permitting them to feed upon this second horse met with failure. The virus which circulated in the blood of the inoculated horse titered as high as 10-5.5, a level not commonly attained in horses. The horse infected by mosquito bite had a lower, more usual concentration of virus in its blood, inadequate to infect mosquitoes. The results of these studies prove it possible for an occasional horse to serve as an EEE infection source for mosquitoes. It is believed, however, that horses rarely play an important role in EEE propagation.